Yet we have occasional glimpses of the method. Natural rafts of uprooted trees and matted vegetation have frequently been seen adrift at sea, more than a thousand miles off the mouths of such great tropical rivers as the Congo, the Ganges, the Amazon, and the Orinoco. Such rafts could easily carry an assortment of insect, reptile, or mollusk passengers. Some of the involuntary passengers might be able to withstand long weeks at sea; others would die during the first stages of the journey. Probably the one best adapted for travel by raft are the wood-boring insects, which, of all the insect tribe, are most commonly found on oceanic islands. The poorest raft travelers must be the mammals. But even a mammal might cover short interisland distances. A few days after the explosion of Krakatoa, a small monkey was rescued from some drifting timber in Sunda Strait. She had been terribly burned, but survived the experience.
No less than the water, the winds and the air currents play their part in bringing inhabitants to the islands. The upper atmosphere, even during the ages before man entered it in his machines, was a place of congested traffic. Thousands of feet above the earth, the air is crowded with living creatures, drifting, flying, gliding, ballooning, or involuntarily swirling along on the high winds. Discovery of this rich aerial plankton had to wait until man himself had found means to make physical invasion of these regions. With special nets and traps, scientists have now collected from the upper atmosphere many of the forms that inhabit oceanic islands. Spiders, whose almost invariable presence on these islands is a fascinating problem, have been captured nearly three miles above the earth’s surface. Airmen have passed through great numbers of the white, silken filaments of spiders’ ‘parachutes’ at heights of two to three miles. At altitudes of 6000 to 16,000 feet, and with wind velocities reaching 45 miles an hour, many living insects have been taken. At such heights and on such strong winds, they might well have been carried hundreds of miles. Seeds have been collected at altitudes up to 5000 feet. Among those commonly taken are members of the Composite family, especially the so-called ‘thistle-down’ typical of oceanic islands.
An interesting point about transport of living plants and animals by wind is the fact that in the upper layers of the earth’s atmosphere the winds do not necessarily blow in the same direction as at the earth’s surface. The trade winds are notably shallow, so that a man standing on the cliffs of St. Helena, a thousand feet above the sea, is above the wind, which blows with great force below him. Once drawn into the upper air, insects, seeds, and the like can easily be carried in a direction contrary to that of the winds prevailing at island level.
The wide-ranging birds that visit islands of the ocean in migration may also have a good deal to do with the distribution of plants, and perhaps even of some insects and minute land shells. From a ball of mud taken from a bird’s plumage, Charles Darwin raised eighty-two separate plants, belonging to five distinct species! Many plant seeds have hooks or prickles, ideal for attachment to feathers. Such birds as the Pacific golden plover, which annually flies from the mainland of Alaska to the Hawaiian Islands and even beyond, probably figure in many riddles of plant distribution.
The catastrophe of Krakatoa gave naturalists a perfect opportunity to observe the colonization of an island. With most of the island itself destroyed, and the remnant covered with a deep layer of lava and ash that remained hot for weeks, Krakatoa after the explosive eruptions of 1883 was, from a biological standpoint, a new volcanic island. As soon as it was possible to visit, scientists searched for signs of life, although it was hard to imagine how any living thing could have survived. Not a single plant or animal could be found. It was not until nine months after the eruption that the naturalist Cotteau was able to report: ‘I only discovered one microscopic spider—only one. This strange pioneer of the renovation was busy spinning its web.’ Since there were no insects on the island, the web-spinning of the bold little spider was presumably in vain, and except for a few blades of grass, practically nothing lived on Krakatoa for a quarter of a century. Then the colonists began to arrive—a few mammals in 1908; a number of birds, lizards, and snakes; various mollusks, insects, and earthworms. Ninety percent of Krakatoa’s new inhabitants, Dutch scientists found, were forms that could have arrived by air.
Isolated from the great mass of life on the continents, with no opportunity for the crossbreeding that tends to preserve the average and to eliminate the new and unusual, island life has developed in a remarkable manner. On these remote bits of earth, nature has excelled in the creation of strange and wonderful forms. As though to prove her incredible versatility, almost every island has developed species that are endemic—that is, they are peculiar to it alone and are duplicated nowhere else on earth.
It was from the pages of earth’s history written on the lava fields of the Galapagos that young Charles Darwin got his first inkling of the great truths of the origin of species. Observing the strange plants and animals—giant tortoises, black, amazing lizards that hunted their food in the surf, sea lions, birds in extraordinary variety—Darwin was struck by their vague similarity to mainland species of South and Central America, yet was haunted by the differences, differences that distinguish them not only from the mainland species but from those on other islands of the archipelago. Years later he was to write in reminiscence: ‘Both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on earth.’
Of the ‘new beings’ evolved on islands, some of the most striking examples have been birds. In some remote age before there were men, a small, pigeonlike bird found its way to the island of Mauritius, in the Indian Ocean. By processes of change at which we can only guess, this bird lost the power of flight, developed short, stout legs, and grew larger until it reached the size of a modern turkey. Such was the origin of the fabulous dodo, which did not long survive the advent of man on Mauritius. New Zealand was the sole home of the moas. One species of these ostrich-like birds stood twelve feet high. Moas had roamed New Zealand from the early part of the Tertiary; those that remained when the Maoris arrived soon died out.
Other island forms besides the dodo and the moas have tended to become large. Perhaps the Galapagos tortoise became a giant after its arrival on the islands, although fossil remains on the continents cast doubt on this. The loss of wing use and even of the wings themselves (the moas had none) are common results of insular life. Insects on small, wind-swept islands tend to lose the power of flight—those that retain it are in danger of being blown out to sea. The Galapagos Islands have a flightless cormorant. There have been at least fourteen species of flightless rails on the islands of the Pacific alone.
One of the most interesting and engaging characteristics of island species is their extraordinary tameness—a lack of sophistication in dealings with the human race, which even the bitter teachings of experience do not quickly alter. When Robert Cushman Murphy visited the island of South Trinidad in 1913 with a party from the brig Daisy, terns alighted on the heads of the men in the whaleboat and peered inquiringly into their faces. Albatrosses on Laysan, whose habits include wonderful ceremonial dances, allowed naturalists to walk among their colonies and responded with a grave bow to similar polite greetings from the visitors. When the British ornithologist David Lack visited the Galapagos Islands, a century after Darwin, he found that the hawks allowed themselves to be touched, and the flycatchers tried to remove hair from the heads of the men for nesting material. ‘It is a curious pleasure,’ he wrote, ‘to have the birds of the wilderness settling upon one’s shoulders, and the pleasure could be much less rare were man less destructive.’